Detecting and locating the approaching enemy aircraft during a war is integral in making sure that the troops are prepared in case of enemy assault, regardless of what they had with them— bombs, chemical weapons, maybe paratroopers. It’s great that radar (radio detection and ranging) was invented, thus, making it easier for soldiers to locate exactly where these planes are. Before it was made, however, people had to rely on what was available for us to use: eyes and ears. Just like any other primitive ways that our ancestors used in their time, experiments were done, too, to enable humans to do beyond what our bodies allowed us.
Before radar and all the other machinery inventions before it, the capability of detecting aircraft during the times of war was the responsibility of human spotters. They were often positioned in open fields, shorelines, rooftops of tall buildings, and hills so they could monitor and spot approaching enemy aircraft and send warnings. However, the effectiveness of this method was reliant on many parameters: the eyesight and hearing quality of the observers, their alertness, the visibility of the surrounding atmosphere (if it was raining or foggy), the level of light, as well as the size, color, configuration, and noise level of the aircraft.
Assuming it was the perfect and most ideal conditions and the human spotter was able to detect the aircraft as soon as humanly possible, it would only allow a few minutes for the soldiers to prepare a response before the plane reached their position. Usually, observer networks were placed up far in advance, and information was transmitted through a radio relay network, but the method was still not always reliable.
From mid-World War I until the early years of World War II, the acoustic location was used for passive detections of aircraft by picking up the noise of their engines. How passive acoustic location worked was that sound or vibrations created by the detected object were analyzed to determine its location. At the same time, horns were used to increase the observer’s ability to localize the direction of the sound. These techniques, during that time, had the advantage as sound refraction allowed them to “see” around corners and over hills.